The 1250 and the 1305 have different grades of bristle, though I couldn't say how different they are. I especially like the bristle in the 1305 and the denser 830 (same bristle as the 1305). I'm not sure if this exact same bristle is used in any other brush by Semogue.

After using the Semogue 1305 for several months now, I've concluded that the Omega 11137 is a better brush for several reasons:
1. More bristles due to wider knot
2. And for reason #1, holds more water and makes better lather
3. The bristles (no doubt due to density) have a little more backbone
4. The handle feels some better in my hand, but that's not a major difference.
5. The Omega costs more but to me that's marginal for my likes over the Semogue.